Optical fiber transmission systems are being extensively used in the telephone network for long distance and interoffice trunk lines because of their wide bandwidth, small size and insensitivity to electrical interference. Conventional long distance optical transmission system utilize time division multiplexed digital transmission. The maximum data rate available in commercial lightwave systems was for many years limited to 565 megabits per second, and has only recently been increased to 1.7 gigabits per second. A 565 megabits per second optical trunk line carrying 8,000 voice channels is very cost effective for voice transmission.
Recently, efforts have been made in the telecommunications industry to utilize optical transmission systems in the local, or subscriber, loop between the central office and individual subscribers. The goal is to provide not only voice, but also data and video transmission over the optical fiber to every home and business. The video services are expected to include not only broadcast services, but also switched video services which will enable each subscriber to select programming and movies from video libraries. An uncompressed digital video signal requires a data rate of about 100 megabits per second, and analog FM video requires a bandwidth of about 30 megahertz. As a result, the 565 megabit per second system, which is so effective for carrying voice channels, carries only a few video channels and must be supplemented with extensive video switching capability just to equal the channel selection presently available on cable TV. While optical fibers, laser diodes and photodiodes have more than adequate capability for bandwidths in excess of 565 megabits per second, the limiting factor is the unavailability of high speed digital electronics that are required for transmitters, for receivers and for multiplexing and demultiplexing circuits. To compete with conventional cable TV, which can provide 30 or more video channels, a subscriber distribution network based on conventional baseband digital fiber optic transmission must either operate at multigigabit per second data rates, or require extensive video switching capability.
To overcome these difficulties, microwave multiplexing of optical signals has been proposed. In these systems, a wideband microwave signal composed of many frequency multiplexed microwave carriers is used to intensity modulate a high speed laser diode. The optical signal is transmitted through a conventional single mode optical fiber to a remote location. The optical signal received at the remote location is detected with a high speed photodiode, and the transmitted signals are recovered with conventional microwave electronics. The microwave carriers can be modulated by either analog or digital signals and can be used to carry voice, data, video, digital audio, and high definition video, in almost any combination of services. Microwave modulated optical systems can be designed to transmit 4-8 gigahertz of bandwidth and can utilize the low-cost equipment presently utilized for satellite video transmission. Transmission of 60 frequency modulated video channels over 18 kilometers of optical fiber is described by R. Olshansky et al in "60-Channel FM Video Subcarrier Multiplexed Optical Communication System," Electronics Letters, Vol. 23, No. 22, pages 1196-1198 (Oct. 22, 1987). The transmission of ten FM video channels over 35 kilometers of optical fiber is described by W. I. Way et al in "A 1.3-.mu.m 35-km Fiber-Optic Microwave Multicarrier Transmission System For Satellite Earth Stations," J. Lightwave Technol., Vol. LT-5, No. 9, September 1987, pages 1325-1332. The transmission of three 44 megabit per second signals over two kilometers of optical fiber is described by T. E. Darcie et al in "Lightwave System Using Microwave Subcarrier Multiplexing," Electronics Letters, Vol. 22, No. 15, pages 774-775 (July 17, 1986). An optical local area network utilizing microwave modulation of a light beam is disclosed in U.S. Pat. No. 4,701,904 issued Oct. 20, 1987 to Darcie.
In order to provide a wide range of subscriber services, it is desirable to optimize the information-carrying capability of the optical transmission system, while maintaining high quality video transmission and low error rate digital transmission. One factor that affects both the information-carrying capability of the lightwave and the noise level or error rate is the modulation index. Each microwave carrier produces a predetermined intensity modulation of the light beam. It is known that the signal-to-noise ratio is improved by increasing the modulation index in each channel. However, when a number of microwave carriers are transmitted simultaneously, the modulation of the light beam by the individual carriers produces a composite intensity modulation that is larger than the modulation index of each channel. It has been thought necessary to maintain the total modulation index at no greater than about 25% to 35% to prevent the laser diode from becoming biased below threshold and introducing significant errors in the transmitted signal. Consequently, it was thought necessary when transmitting many microwave subcarriers simultaneously to limit the modulation index of each to a very low value and/or to limit the number of channels transmitted. In the above-referenced article by W. I. Way et al, optical modulation indices of 2% per channel or less were utilized for ten channels, resulting in a total modulation index of 20% or less.
It is a general object of the present invention to provide improved optical communication systems.
It is another object of the present invention to provide an optical communications system wherein a light beam is modulated by a plurality of microwave carriers, and the total modulation index exceeds unity.
It is a further object of the present invention to provide optical communication systems having a very large information-carrying capability.
It is still another object of the present invention to provide optical communication systems having the capability to carry voice, data and multiple video channels.